In an image reading device used in a digital multifunction peripheral, in order to read an image for one line extending in a main scanning direction, linear light extending in the main scanning direction is irradiated from a light source to an original document and reflected light from the original document is condensed by a lens and made incident on an image sensor.
When the light from the light source is made to reach the original document, a prism may be used. The prism is received in a case. When the linear expansion coefficient of the prism is larger than the linear expansion coefficient of the case, there is a concern that the prism may collide against the case due to thermal deformation of the prism. When the prism is curved, the illumination characteristics for the original document may be badly influenced.